Page 105 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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Doppler flow meter.
FIGURE 4.25

& Multipath ultrasonic flow meters use several sets of either transit time or Doppler flow meters to work
sending and receiving transducers placed in different properly.
paths placed across the flow section, to determine the & The use of multipath flow meters in raw wastewater
flow rate. Most multipath flow meters use four to six and storm water applications is common, while
different paths or ultrasonic signals to determine the Doppler or cross-correlation hybrid designs are most
velocity profile across the entire cross section of the often used to measure activated sludge and digested
pipe. The transducers alternate sending and receiving sludge flows.
a signal over the same path length. Flow rate is
& For mining slurries, Doppler flow meters work well.
determined by averaging the values given by the
& Among the few problem applications are those in
different paths, yielding greater accuracy than pro-
HDPE pipe because the pipe wall flexes enough to
vided by single-path meters.
change the diameter of the measurement area. This
& Multipath instruments are used in large diameter
affects the accuracy of the meter.
conduits, such as stack gas flows in power plant
& Another problem area is the measurement of slurries
scrubbers and in other applications where nonuni-
that are acoustically absorbent, such as lime or kaolin
form flow velocity profiles exist.
slurries. These applications fail because the highly
. What are the applications of ultrasonic flow meters?
absorbent solids attenuate the signal below usable
& Ultrasonic flow meters are used to measure the flow
strength.
of both liquids and gases. & These meters must be calibrated for the fluid flowing
& Transit time flow meters are mainly used for clean,
through them.
swirl-free liquids and gases of known profile. Having . Will pipe thickness or insulation affect readings of a
clean fluid is the most important constraint on ultra-
Doppler flow meter?
sonic flow meters although transit time meters can
& Yes. Insulation should be removed before mounting
handle some impurities.
the sensor.
& They can be affected by swirl.
. Does a Doppler flow meter require a minimum upstream
& It is important to take into consideration both pipe
straight pipe length?
and fluid characteristics into account.
& Yes. Doppler flow meters require 10 pipe diameters
& Doppler ultrasonic flow meter requires the presence
from any valve, tee, bend, and so on.
of particulates or bubbles in the flow stream so the
& Doppler flow meters also require a full pipe flow.
signal can bounce off them. Hence, they are used with
. What are the advantages and the disadvantages of using
unclean liquids and slurries and not recommended
a Doppler flow meter?
for clean fluids. Minimum size of particulates in the
Advantages
stream should be 30 mm and concentration of parti-
culates should be >25 ppm. & The main advantage of the Doppler ultrasonic meter
is its nonintrusive design. An acoustic coupling
& Raw wastewater-related applications are problemat-
compound is used on the surface of the pipe and the
ic, as the solids concentration can be too high for